I think how this would proceed in practice is dependent upon what the the person nominating the JW for 'targeting' thinks is the 'weak spot' issue. They know this best because they know the JW concerned, so they can instruct the undercover 'interested one' about which issues to target.
For instance, they may know that honesty is particularly important to this JW. So they might say that the Society's dishonesty over the history and past teachings of the Org might be the place to start.
So, an undercover "interested one", having made contact with the JW, may say "OK, I am interested in learning about the Bible, but I'd like to know a little more about your Organization first so I know who I'm studying with and what I'm getting into. So tell me a little about your group. How did you get started?"
Maybe that could introduce the subject of the Society's history and the justification for addressing the topic. In discussing the Organizations history, a JW is bound to mention Russell and try to suggest what made Russell and his 'Society" special. 1914 is bound to come up. And the JW could be drawn into (unknowingly) repeating the Society's lies about what they taught about 1914 before 1914.
Then the "interested one" could say something like "Wow. So, Russell preached that Jesus would become King and be present from 1914, ousting Satan from heaven, and then the World War broke out in 1914 as evidence that this had happened, just as he said? That's quite a prediction! That really happened? Wow. I don't mean to be rude, but can you prove that? I'd love to see the books where he said that. That would be quite some evidence that Russell and the JW's were on to something and have God's backing, I agree."
Then the JW is obliged to prove it and do research, perhaps in the old books in the hall. Meanwhile the "interested one" can be doing their own "research" into the old literature: "Oh, by the way. I really enjoyed our discussion about the early JW's and Russell and what they said. I found one of his old books on Ebay and I bought it." Then this can be shared when they come together. Quickly, it should become apparent that the facts don't tally with what the JW had said. The "interested one" would not be accusatory and challenging, but simply puzzled that things don't seem to add up - much as the JW will likely be.